This invention relates to brushless D.C. motors and, more particularly, to such motors employing permanent magnet rotors and commutation circuits controlled by Hall effect elements.
Conventional D.C. motors, employing segmented commutators and brushes to achieve the polarity switching necessary for rotation, present certain obvious shortcomings. The wear on brushes and commutator segments necessitates periodic maintenance and/or replacement and the sparking occurring between the brushes and commutator segments produces undesirable radio disturbances. Moreover, the sparking presents a hazard where the motor is exposed to inflammable or explosive gases.
To avoid the disadvantages of mechanical commutation, a number of commutatorless systems for D.C. motors have been devised over the years. Basically, these systems employ some means for detecting or responding to rotation of the rotor to switch currents through the stator windings, so that the polarity of the latter are periodically reversed to maintain rotation. With the advent of solid state technology, it has been possible to reduce the physical size of the required circuitry such that it may be incorporated in the motor without any appreciable increase in overall size of the structure.
In one commercial form of brushless D.C. motor, a permanent magnet rotor is used and the rotation of the magnets is sensed by Hall effect elements. A Hall effect element, or Hall cell, is a low-power semi-conductor device, current flow through which can be altered by magnetic flux to produce a voltage output across a pair of output electrodes. The greater the magnetic flux density to which it is exposed, the greater the voltage output developed.
In these known motors employing Hall effect devices, the Hall effect devices are generally exposed to the magnetic fields generated by the permanent magnet rotor and the stator poles and complex circuitry is provided to sense the potential output of the Hall devices and generate the driving currents for the stator windings. Because of the normal response of Hall effect devices, these prior art motors require sophisticated mechanical adjustments to the rotor and/or stator structure to insure constant speed rotation of the rotor. These modifications may take the form of additional ferromagnetic members on the stator structure for the purpose of interacting with the rotor magnets to provide increments of torque in the gaps between energization of the stator windings. In another form, the air gap between the stator and rotor gradually increases and then decreases across each stator pole face for the purpose of storing and then releasing magnetic energy, to supply torque between periods of energization of the stator coils.
In other prior art systems, complex mounting arrangements for the Hall effect devices are necessary to expose them to the magnetic flux from both the rotor magnets and the stator poles so that they counter each other and reduce voltage peaks through the driving transistors for the motor coils, thereby smoothing operation of the motor.
Another problem inherent in prior art D.C. brushless motors is difficulty in starting rotation of the permanent magnet rotor, since the rotor tends to seek a rest position at the lowest reluctance point when the motor is shut off.